Today, processors and system-on-a-chip (SoC) designs operate on multiple power supply levels. For example, input-output circuits (I/Os) generally operate at higher power supply levels than the core of the processor because I/Os may require higher voltage level swings to transfer data across transmission lines. The core of the processor generally operates at a lower power supply level to save power dissipation where most of the circuits reside. Similarly, Phase Locked Loops (PLLs) require quiet power supply levels that are shielded from the noise generated on the power supply levels from the core of the processor. In multi-core processors, each core may operate on a different power supply domain resulting in a complex system of multiple power supply domains between cores and within cores.
For circuits operating on one power supply domain to communicate with a circuit on another power supply domain, interface circuits are used. These interface circuits are called level-shifters.